It is a common practice to use the adsorption technique, on adsorbent beds of materials like activated carbons, macroporous resins, molecular sieves, zeolites and activated alumina, to separate volatile organic compounds (VOCs), as well volatile inorganic compounds, which are present as pollutants in gas emissions of industrial processes and, for process reasons, in liquid or gas mixtures which are formed in many industrial production processes.
It is also known to regenerate adsorbent materials to allow reuse thereof in several cycles, by means of desorption and recovery of the volatile compounds released from the adsorbent material, for reuse or disposal thereof.
For such purpose, adsorbent materials must be heated by convection (using steam or other fluids), or by conduction (indirect heat exchange), or by radiation (for example, using microwaves), to supply the adsorbed compounds with the energy needed for desorption thereof.
Although the use of steam is effective, it often causes partial hydrolysis of the compounds to be desorbed, and serious corrosion problems, and at all events the formation of huge amounts of condensed water mixtures to be processed to allow reuse or disposal thereof.
Indirect heat exchange, although associated with vacuum, requires unacceptable operation times for industrial applications, as adsorbent materials typically have a poor heat transfer coefficient.
U.S. Pat. No. 5,779,768 (ANAND) discloses regeneration of adsorbent materials by using a stream of hot inert gas, e.g. nitrogen, which passes through the material to be regenerated.
The gas stream provides a sufficient amount of heat energy for the adsorbed compounds to break their bonds with the adsorbent materials, and the hot stream strips the compounds released from the adsorbents, in form of vapors.
In steady conditions, the gas enriched in desorbed compounds is delivered, totally or partially, to a condenser where it is cooled to a temperature below the dew point, to obtain condensation of the adsorbed compounds (except a small portion that remains in the vapor phase), for reuse or disposal thereof.
According to this prior art technique, throughout the process, the whole gas stream must be simultaneously heated (upstream from the bed of material to be regenerated) and cooled (downstream from the bed); this also occurs in other prior art techniques.
When considering the high flow rates (usually at least twice the adsorption flow rates) that are needed to carry out regeneration in short times, the high temperatures required for regeneration of the adsorbent material and, conversely, the low cooling temperatures that are needed to condensate the separated volatile compounds, it is apparent that this prior art technique requires the simultaneous supply of huge healing and cooling powers to the gas stream used for desorption and recovery of volatile compounds.
Therefore, this prior art technique requires complex and expensive systems, particularly requiring high running costs, whose increase is inversely proportional to the cooling temperature required to obtain a good regeneration of the adsorbent material.
Also, it might produce an unsatisfactory level of adsorbent material regeneration, when compared with the increasingly strict emission limits prescribed in many countries for environment protection.